1. Field of the Invention
This invention relates to an image processing apparatus such as a printer, a copier, a facsimile, or a multifunction apparatus which has one or more such functions, and which has an image overlaying function.
2. Discussion of the Background
In recent years, in an image processing apparatus, the image editing function using an image memory has become popular in the trend towards digitization. For example, the "image rotation" function and the "image repeating" function implemented by an addressing at a reading of a memory, and the "into one" function by which two or more originals are formed on one recording paper by an addressing at a reading of a memory are known. Generally, the copier which has these function has more than one page image memory.
Also known is the "image overlaying" function which also uses the image memory. In this image overlaying function, two images are read by respective scannings and the two read images are overlaid, and thereafter outputted.
As a method of image overlaying, three methods are known.
(a) At first, two images which are read by scanning are stored in an image memory. Next, two stored images are overlaid and the overlaid image is stored in the image memory. Then, the overlaid image is outputted.
(b) At first, two images which are read by scanning are stored in an image memory. Then, these two images are read out from the image memory in parallel with overlaying and this overlaid image is outputted.
(c) At first, one of two image is scanned and stored an image memory. Next, an other image is scanned. The stored image is read out from an image memory in parallel with this scanning of the other image and these image are overlaid and outputted.
Method (a) and (b) are proposed by Japanese Patent Laid-open No. 61-176279 and method (c) is proposed by Japanese Patent Laid-open No. 60-114080. In these three methods, methods (a) and (b) need an image memory of more than two pages because in these methods, a storing of two page images is necessary. A large memory however increases cost of the apparatus.
On the other hand, method (c) needs an image memory of only one page. But method (c) has several problems. In particular, in method (c), the synchronization by pixel between the scanning and the reading out from the image memory is needed because the scanning and the reading out are done in parallel. Therefor, the reading out is done in sync with the PCLK (pixel clock) signal from a scanner. Because all processing must be done based on the PCLK from a scanner, a process speed is determined according to the speed of scanner. For example, during the scanner return, image forming is not able to be carried out. This is a first problem.
Recently, there is a digital copier which has some functions of a printer, a facsimile and a copier. In this digital copier, while outputting a copy image from an image memory, a sending facsimile from a scanner is possible. However, in the method (c), while outputting an overlaying image, a sending facsimile is impossible because a scanner is used for reading an image. This is a second problem.
A third problem concerns a memory copying function for forming of more than two sheets according to one original. In the memory copying function, when an original image is read by scanning, the image is formed on the first sheet and the read image is stored an image memory. Then, the second and later sheets are formed using the stored image without scanning. Therefore, copying speed is normally increased because a returning time of a scanner is not necessary.
However, in image overlaying function by the method (c), a scanning is necessary for forming the second or later sheet also because a scanning is necessary for image overlaying. Therefore, in the method (c), a memory copying is impossible.